Semiconductor display devices, such as liquid crystal display devices and EL display devices, used in portable electronic devices have been required to reduce an area other than a pixel portion (i.e., to narrow a frame width). A system-on-panel obtained by forming a pixel portion and some of or all driver circuits over one substrate is effective in meeting the above need. In the case of the system-on-panel, like the pixel portion, the driver circuit preferably includes unipolar transistors because the panel can be manufactured at low cost. Patent Documents 1 and 2 disclose techniques for formation of various circuits such as inverters and shift registers that are used in driver circuits of semiconductor display devices and include unipolar transistors.